Manufacturers of electronic equipment frequently aim to make their equipment as compact as possible. To do that, the circuit boards or modules that form the inner workings of the electronic equipment must also be made compact. This latter requirement can be met by using flexible, polyimide substrates (sometimes referred to as "flex circuits") on which individual circuit components are mounted. The ability to bend such flex circuits without damage allows them to be assembled in relatively small packages compared to the size of the packages required to hold conventional rigid printed circuit modules. A publication entitled "Flexible Circuitry Design Guide" from Sheldahl, Inc. (Copyright 1984) illustrates the use of such flex circuits.
Another factor which must be considered in designing a compact assembly for electronic circuit modules is the extent to which the assembled circuit modules need to be shielded from EMI (electro-magnetic interference). Such EMI may be generated by other nearby electronic equipment or by one of the assembled circuit modules. In either case, the need to provide shielding from EMI usually results in a large, less compact assembly and a correspondingly larger housing for the assembly.
The problem is compounded in the case where one or more of the enclosed electronic modules requires a relatively massive heat sink. To enclose one or more of such heat sinks with the circuit modules, while also providing EMI shielding, has required an undesirably large housing.